1. Field of the Invention
The present invention relates to a variable gain amplifier device configured to vary a gain exponentially with respect to a gain control signal, applicable to a wireless communication apparatus such as a portable wireless device.
2. Description of the Related Art
Portable wireless devices such as a portable telephone are required to be reduced in size and weight so that they can be carried by a man, or mounted on a car or the like for use. Therefore, parts constitute the devices have been strongly desired to be monolithic ICs (integrated circuit) suitable to reductions in size and weight as compared with the hybrid construction by connecting a plurality of discrete elements. Monolithic ICs also contribute to a cost reduction of the devices.
As one of the configuration methods of a wireless transmission/receiving circuit in a wireless communication apparatus such as a portable wireless device, the direct conversion system is known. According to the direct conversion system, on the transmission circuit side, two transmission baseband signals in the quadrature relation are amplified respectively by a variable gain amplifier. The amplified quadrature transmission baseband signal is input to a quadrature modulator and converted directly into a radio-frequency (RF) signal. The RF signal is amplified by a power amplifier and transmitted by an antenna. On the reception circuit side, a reception RF signal supplied from the antenna is amplified by a low noise amplifier, and then converted directly into two reception baseband signals in the quadrature relation by a quadrature demodulator (down converter). The reception baseband signal is demodulated by a baseband signal processor.
In the field of portable telephones, recently, a CDMA (Code Division Multiple Access) system capable of high speed and high capacity communication is noted. The CDMA system performs a communication by multiplexing transmission/receiving data of a plurality of users by spreading respectively with different codes on a carrier of an identical frequency. The CDMA system cannot perform correct communication if there is a large dispersion in the power of the transmission data from a plurality of users carried by a carrier of the identical frequency received by a base station. Consequently, in a portable terminal, it is essential to perform a wide transmission power control range such as, for example, 70 dB or more, according to the distance from the terminal to the base station.
In a wireless communication apparatus using the direct conversion system, it is necessary to provide the baseband section with a variable gain amplifier in order to realize such a wide range of transmission power control. This is because it is difficult to realize the wide transmission power control range only by the RF section since the variable gain range in the RF section is limited by the isolation between input and output.
U.S. Pat. No. 6,215,989 discloses an example of a variable gain amplifier capable of a wide range gain control. According to the variable amplifier, a first gain control signal Vc is converted into a second gain control signal Vy by a control signal converter. The control signal Vy is input to a base terminal of bipolar transistors Q1, Q2 forming a differential pair in a current dividing type gain controlled amplifier. The transfer function of the gain controlled amplifier can be expressed by the Equation (1):Iout/Iin=1/[1+exp(Vy/VT)]  (1)
Here, Iin and Iout are input signal current and output signal current of the gain controlled amplifier, respectively, and VT is thermal voltage, which is about 26 mV at normal temperature.
On the other hand, the transfer function of the control signal converter is expressed by the Equation (2):Vy=VT·ln{exp(b·Vc/VT)−1}  (2)where, b>=0. Vc>=0. In the case Vc=0, from the Equation (2), Vy becomes −∞, which is interpreted that the input signal current Iin flows only in one transistor Q1 of the differential pair.
From the Equations (1), (2), a total gain of the variable gain amplifier including the control signal converter can be expressed by the Equation (3):
                                                                        I                ⁢                                                                  ⁢                out                ⁢                                  /                                ⁢                I                ⁢                                                                  ⁢                in                            =                            ⁢                              1                ⁢                                                      /                                    ⁡                                      [                                          1                      +                                              exp                        ⁡                                                  (                                                      Vy                            ⁢                                                          /                                                        ⁢                                                          V                              T                                                                                )                                                                                      ]                                                                                                                          =                            ⁢                              1                ⁢                                                      /                                    [                                      1                    +                                          exp                      ⁡                                              [                                                                              V                            T                                                    ⁢                                                      /                                                    ⁢                                                                                    V                              T                                                        ·                            n                                                    ⁢                                                      {                                                                                          exp                                ⁡                                                                  (                                                                                                            b                                      ·                                      Vc                                                                        ⁢                                                                          /                                                                        ⁢                                                                          V                                      T                                                                                                        )                                                                                            -                              1                                                        }                                                                          ]                                                                                                                                                                    =                            ⁢                              1                ⁢                                                      /                                    ⁡                                      [                                          1                      +                                              exp                        ⁡                                                  (                                                                                    b                              ·                              Vc                                                        ⁢                                                          /                                                        ⁢                                                          V                              T                                                                                )                                                                    -                      1                                        ]                                                                                                                          =                            ⁢                              1                ⁢                                                      /                                    ⁡                                      [                                          exp                      ⁡                                              (                                                                              b                            ·                            Vc                                                    ⁢                                                      /                                                    ⁢                                                      V                            T                                                                          )                                                              ]                                                                                                                          =                            ⁢                              exp                ⁡                                  (                                                                                    -                        b                                            ·                      Vc                                        ⁢                                          /                                        ⁢                                          V                      T                                                        )                                                                                        (        3        )            
From the Equation (3), it can be understood that the gain reduces exponentially if the first gain control signal Vc is increased from 0(V) in the positive direction. In the Equation (1), the denominator 1 in the right side impedes the gain control of the exponential function. According to the Equation (3), the gain varies exponentially because 1 doesn't exist in the denominator of the right side.
In a portable telephone, it is desirable to realize the circuit as much as possible by a CMOS process of low-cost more than the bipolar process. When the aforementioned variable gain amplifier is realized by using the CMOS process, the gain control characteristics shown by the Equation (3) cannot be obtained. This is because the input-output characteristics of the MOS type transistor is a square characteristic as shown by the Equation (5), while the input-output characteristics of the bipolar transistor is an exponential characteristic shown by the Equation (4):Ic=Is·exp(VBE/VT)  (4)Id=β(VGS−VTH)2  (5)where, Is is saturated current, VBE is base to emitter voltage, VT is thermal voltage, β is a constant depending on the size of the MOS type transistor or process, VGS is gate to source voltage and VTH indicates threshold voltage.